1. Field of the Invention
The present invention relates to a system and a method for image signal processing; in particular, it is for a system and a method for contrast extension and overflow compensation of an image signal.
2. Description of the Prior Art
Among the known video image products or components, there are frequent needs for adjusting the quality of images by using image processing. For example, the brightness function, the contrast function, the hue function, and the saturation function designed for a TV set are provided to consumers for adjusting the image quality in accordance with personal preferences. However, the problem that is frequently encountered by consumers is that after the image is adjusted by use of the selections in the functions, the phenomenon of a large area of saturated transparency of the image arises. That is to say, different levels of brightness in the image or the colors previously distinguishable can no longer be distinguished in the original levels of brightness or color deviations after the image or the colors are adjusted to full brightness, causing the phenomena of overflow of the saturated gray scale. That means the gray scale values have reached the maximum value, and their original levels can no longer be distinguished.
In another example, most video processing IC provides many types of video adjusting and enhancing functions for designers to find the most optimum adjustment settings. However, after several levels of processing of the image, the phenomenon of saturated overflow frequently occurs, causing the image ultimately appearing on the display to show a large area of transparency, thus affecting the viewing quality.
Current solutions are mostly carried out as a simple countermeasure on image overflow by limiting image gain value during processing, so as to reduce the possibility of generating image saturation as much as possible. However, image signals always go through more than one level of processing, and if the image signals go through several levels of adjustments and enhancement processing, the number of gray scales of the image decreases, causing the problem where the image continuously has an incomplete contour, and the overall brightness is lowered, thus resulting in a poor contrast.
Furthermore, in order to prevent overflow from occurring, the gain value is usually limited to lower than 1.0 when the image signal is being processed; therefore, after several levels of image processing, the gray scale value of the image becomes smaller and smaller, and the contrast of the image becomes poorer and poorer. For example, the maximum range of the gray scale value of the 256 gray scales of an 8-bit image is 0-255. Due to the fact that the nature of the utilized components, parameter settings, decayed signal transmission, decayed dissection, suppression of noisy signals, and image signal processing can all affect the range of image gray scales in practical applications of video systems, the image gray scales cannot fully utilize the 256 gray scales, causing the dynamic range of the image signal to decrease, the brightness of color levels to lower, and the contrast to be poorer. Thus, the viewing quality is affected.
Accordingly, a scope of the invention is to provide a processing system and a processing method for contrast extension and overflow compensation of an image signal, and more particularly, after the number of gray scales of an image signal has dropped or an overflow has occurred after the image signal has gone through several times of preprocessing, the image signal can be automatically adjusted to prevent the problems described above from happening.